1. Field of the Invention
The present invention relates generally to cleaning semiconductor wafers. More particularly, it relates to cleaning, rinsing, and drying semiconductor wafers after a photolithography process.
2. State of the Art
Processing a semiconductor wafer to fabricate semiconductor devices thereon includes numerous steps to form multiple material layers on the wafer surface. One of the most critical and repeated process sequences in semiconductor device fabrication is photolithography. The photolithography process begins with application of a photoresist material upon a semiconductor wafer, typically using a process such as spin coating. The photoresist layer is then exposed to an activating radiation source through a photomask comprising a desired, repetitive pattern of features for the semiconductor devices being fabricated. The photomask pattern prevents the activating radiation from striking some areas of the semiconductor wafer and allowing the radiation through to strike other areas. Exposure of the photoresist by the activating radiation creates a photo-induced chemical transformation in the exposed areas of the photoresist.
Photoresists, also termed “resists” in the art, can be either positive acting or negative acting. With a positive acting resist, the use of which is much more prominent in modern photolithography techniques, the portions exposed to the radiation are rendered more soluble to a developer solution. By applying the developer solution after exposure, the exposed areas of the resist are removed, revealing the underlying surface of the wafer for further processing steps such as ion implantation, material deposition, or etching. After developing, but before moving on to these subsequent processing steps, it is critical to thoroughly clean, rinse, and dry the semiconductor wafer. This photolithography process is typically repeated numerous times to create the various layers involved fabricating semiconductor devices on a wafer. In a negative acting photoresist, portions thereof that are exposed to radiation are those that remain on the wafer to mask portions thereof after processing with a developer solution. However, exposure, development, cleaning, rinsing, and drying of the negative photoresist is effected in a manner similar to that for a positive photoresist.
As semiconductor geometries shrink, the photolithography process has evolved to accommodate smaller semiconductor device feature sizes. Conventionally, photoresist exposure has been accomplished with ultraviolet (UV) radiation. However, as feature sizes approach the wavelength of the exposure radiation, the use of alternative, shorter wavelength light such as deep UV, as well as other types of radiation beams, has been required. As the type of radiation changes, the photoresist composition must change such that it is sensitive to the new radiation wavelengths. Such composition changes may result in a photoresist that is more brittle.
Additionally, as semiconductor device feature sizes continue to shrink, the thickness at which the photoresist can be applied to the wafer has not shrunk proportionally. As a result, after developing and cleaning of a wafer, the photoresist features remaining on the wafer have an increased, or higher, aspect ratio, exhibiting heights that are substantially greater than widths. These types of feature configurations create an enhanced susceptibility to a phenomenon known as “toppling,” which has been manifested to an unacceptable degree in minimum line-space pattern 248 nm and 193 nm photolithography. Lateral stresses placed on a tall photoresist feature comprising, for example, a line can cause the feature to break near the base of the line and topple over. Toppling thus moves the location of the photoresist feature and covers a portion of the semiconductor wafer that was not intended to be covered such that a future processing step such as material deposition will not occur where desired. Additionally, the toppled photoresist feature may break off completely from the desired location and float to a different location on the wafer, causing further processing errors. Unfortunately, one of the most serious sources of stress that can act on these high aspect ratio photoresist features is surface tension of a drying rinsing solution used to clean the wafer of developer solution, during spin out of a wafer. As the rinsing solution dries, the surface tension thereof between adjacent high aspect ratio features such as lines tends create a shear stress which induces a break between the photoresist and substrate (semiconductor wafer) at the resist/substrate interface, pulling the features over.
It has been discovered that surfactants may be used to help reduce surface tension of a rinsing solution. However, while this is effective to reduce toppling, long chain molecule surfactants (which appear to be more effective in reducing surface tension) may be harmful to subsequent etch processes and can leave a residue on the photoresist material as the rinsing solution dries, while short chain molecule surfactants may cause unwanted etching of the photoresist. As a result, while surfactants may be used in conjunction with the present invention, surfactants alone are not sufficient to address the surface tension and toppling problem attendant to the drying process.
A technique employing the well-known Marangoni effect is used in rinsing and drying processes to remove, or “rip” particles from the semiconductor wafer. The Marangoni effect manifests itself because a liquid of a lower surface tension is attracted by a liquid of a higher surface tension. However, in these conventional techniques, the Marangoni effect is used to displace the rinsing solution and replace it with the lower surface tension liquid. In effect, it forces the rinsing solution off the surface being cleaned so that only the lower surface tension liquid remains on the surface. The reason for performing this operation has typically been to remove the rinsing solution and replace it with a liquid that leaves fewer residue markings on the semiconductor wafer. However, in the process of displacement of the rinsing solution, forces similar in magnitude to the surface tension forces associated with drying of rinsing solutions may be applied to the photoresist patterns.
A method is needed to reduce rinsing fluid surface tension to reduce shear stresses imposed on high aspect ratio photoresist features as the fluid dries and thereby minimize toppling of high aspect ratio photoresist features.